Cleaning out solvent from contaminated air by means of adsorption and subsequent desorption of matter(s) is a well known engineering practice. As adsorbent material, activated carbon is often used. To regenerate the activated carbon, the temperature is raised and volatile solvents evaporate. For heating purpose a direct inlet of a hot gas stream which can be condensable, e.g. steam, or non-condensable, e.g. nitrogen, can be used. Heating can also be caused indirectly via heating surfaces, at which the means of heating will be kept separate from the adsorbent material.
Adsorption beddings can be solid, which is the most common construction. During continuous operation, the air must change over to a new bedding when the adsorption capacity of the working bedding is exceeded. The adsorption bedding can also be rotating and segmented, thus air to be purified passes through one segment while strip gas passes through another segment. In this manner, a simultaneous purification of contaminated air and a regeneration of the adsorbent is taking place. A drawback with this construction is that it is expensive and bulky.
At desorption in a solid bedding with s hot, non-condensable strip gas a heat front with steep temperature gradient will slide through the bedding. If the gas flow is uniform the heat front will be parallel to the frontal surface of the bedding, that is, the surface through which the strip gas penetrates into the bedding. In the flow direction, in the front of the heat front, an area will form in which the solvent concentration in the activated carbon increases due to readsorption of desorbate, which has desorbed behind the heat front. Thus, in front of the heat front a concentration wave will form, that is, an area with increased solvent concentration. When this concentration wave reaches the outer surface of the bedding, the concentration in the strip gas leaving the bedding will rise to a level that can be many times higher than the average concentration in the outgoing strip gas during the phase of desorption.
The varying concentration of solvent in the outgoing strip gas will in a construction with solid bedding lead to that air only under certain conditions can be used as strip gas. A risk arises that the solvent concentration, during a certain part of the phase of desorption, can become an explosion hazard. Due to the explosion risk, constructions with solid bedding often require nitrogen or another relatively expensive inert gas to be used as strip gas.
At the same time new laws and regulations have resulted in higher demands for purification of solvent-contaminated exhaust air as well as increasing efficiency of gas purifying equipment. Hence, it is important that apparatuses for cleaning can be made small, effective and inexpensive to run.